A PFBC power plant cannot, after an operational disturbance in the gas turbine, rapidly resume normal operating conditions. The very large potential contents of energy in the PFBC plant pose special problems which are difficult to solve. Since a stop valve in the conduit for the hot gases to the turbine has to be closed to protect the gas turbine, it is necessary to carry off energy from the PFBC plant in such a way that combustion and bed cooling can be controlled.
It has been proposed to allow the hot combustion gases from the bed vessel of a PFBC plant to blow-off to atmosphere. The gases have a temperature of 800.degree.-900.degree. C. and are mixed with about 200 ppm dust. It is difficult--not to say impossible--to cause a valve operating at such a high temperature and in such a severe environment to seal and satisfactorily clean the very large flow of gas at such a high temperature. A suitable valve would be expensive and its working life would be short. A proposal for reducing the inconvenience of leakage in a valve for blowing off combustion gases from the bed vessel is to blow off, at the same time, the compressed combustion air in a pressure vessel surrounding the bed vessel in the manner disclosed in U.S. Pat. No. 4,498,285 (Kreij). The combustion gases are then mixed with air and cooled, so that a valve in the pressure vessel wall is not subjected to gases of such extremely high temperatures. In a valve in the bed vessel wall a certain leakage into the bed vessel may be tolerated. However, this method is also not wholly satisfactory.
Another solution is to have a cut-off valve in the hot gas conduit leading to the turbine with a small leakage and let gas leak through the valve or have a by-pass valve in parallel with the cut-off valve. The problem is that the volume of air flowing through the hot bed of fuel in the bed vessel is reduced to the point where the O.sub.2 content does not suffice for complete combustion of the fuel present in the bed. Combustible gases, e.g. of carbon monoxide (CO), are thus formed and may start burning downstream of the cut-off valve in the gas conduit and give rise to an impermissibly high gas temperature and an impermissibly high gas turbine speed.
There is also a possibility of sinter formation in the bed due to uncontrolled combustion and cooling within the bed volume.